1. Field of the Invention
This invention relates to a sheet size detection device for detecting the size of sheets held in the body of a cassette which can cope with two or more sheet sizes, and more particularly to a sheet size detection device which is applicable to a sheet feeder of an image forming apparatus such as a copier, a printer or a facsimile apparatus.
2. Related Art
In an image forming apparatus such as a copier, generally, a sheet feed cassette which is detachably mounted on the main body of the apparatus is used as a sheet feeder. Such sheet feed cassettes include an exclusive cassette which can accommodate only standard sheets of a predetermined one of the sizes of JIS A series such as A3 or A4, or JIS B series such as B4 or B5, and a universal cassette and can cope with two or more sheet sizes.
When such a universal cassette is used, it is required to indicate the sizes of sheets with which the cassette can cope. Moreover, when an automatic magnification copy operation is to be done, it is necessary to display on the main body of a copier the size or direction of sheets which are held in a universal cassette. An example of a sheet size detection device which detects the size of sheets held in such a cassette is disclosed in Japanese Laid-Open Patent Publication No. Sho 63-185730 entitled "Sheet supply cassette".
In this sheet size detection device, a transverse guide member and feeding-direction guide member which respectively define the position of sheets in the transverse and longitudinal directions are moved, thereby causing sliding contacts of variable resistors corresponding to the members, to slide. On the basis of the variations in resistance, the size of the sheets can be detected.
In this conventional sheet size detection device, however, current levels which vary depending on the position of the feeding-direction guide member and that of the transverse guide member must be independently measured in the longitudinal direction of the sheets (the feeding direction of the sheets) and the transverse direction, so that a control circuit provided in the main body of the copier judges the size of the sheets on the basis of the measured current levels. This brings problems in that the circuit configuration is complex, the control circuit is liable to malfunction, and the costs of parts are high. When the detection device is constructed so as to detect larger sheet sizes, furthermore, a contact failure is liable to occur in the wiring portions, thereby causing a further problem in that the reliability of the detection device is reduced.